Proteins are increasingly being used as pharmaceuticals, diagnostic agents, catalysts in biochemical processes involving the isolation, storage, and use of thiol-containing proteins are hampered by the lack of practical processes to manipulate the oxidation state of protein thiols. Although protein thiols can be manipulated by thiol/disulfide exchange, conventional thiol/disulfide exchange requires adding a stoichiometric amount of expensive and potentially hazardous thiol reagent and is restricted to only a few thiol reagents. This proposal describes a program aimed at demonstrating the feasibility of a novel method to carry out thiol/disulfide exchange that uses a catalytic amount of thiol reagent that is continuously regenerated via a membrane-based process.